In the field of dental treatment, there are used prostheses such as crowns, bridges and inlays to repair or substitute for missing or defective teeth or oral tissues. As materials for adhering these prostheses to teeth, compositions are mainly used that comprise polymerizable resins and filler particles; these compositions cure through such mechanisms as cold (chemical) polymerization and photopolymerization.
To effect cold polymerization of polymerizable compositions that contain radical polymerizable methacrylates, acrylates or other monomers, oligomers, and prepolymers, a commonly used means is a redox-radical polymerization initiator system consisting of an oxidizing agent in combination with a reducing agent. For example, a polymerization initiator in which an organic peroxide as an oxidizing agent is combined with an aromatic tertiary amine as a reducing agent has been used for quite a few years (see, for example, JP H10-338610 A). Under water-free conditions, this initiator presents no problem with the polymerizability of monomers but under wet conditions as in the oral cavity, it has sometimes occurred that the polymerization of monomers is inhibited.
As mentioned above, polymerizable compositions for use in the field of dental treatment need be polymerized under wet conditions as in the oral cavity. To this end, a polymerization catalyst is required that can be exposed to water without suffering a drop in polymerizing activity but there are not many polymerization catalysts that are known to satisfy this requirement. Among the very few exceptions is a polymerization initiator system that consists of a water-soluble reducing agent such as ascorbic acid and a water-soluble oxidizing agent such as persulfate (see, for example, JP H8-500080 A). This initiator system shows satisfactory curing property even in the presence of water. In practice, however, the storage stability of this initiator system is so low that it has been necessary to take the trouble of microencapsulating either the oxidizing agent or the reducing agent or both.
Another problem with the above-described approach is that the polymer containing ascorbic acid undergoes discoloration; to solve this problem, preliminary metal complexing of ascorbic acid has been proposed but this method presents another problem of requiring a very complicated procedure (see, for example, WO 95/22955).
The assignee of the present invention previously developed a polymerization initiator system consisting of cumene hydroperoxide, a thiourea derivative, and a vanadium compound (see JP 2012-51856 A). This system is held to be useful since it ensures not only outstanding curing property but also satisfactory storage stability. However, most thiourea derivatives are extremely toxic and acetylthiourea and N-benzoylthiourea, in particular, which are superior polymerization initiators are highly toxic to humans and, hence, have been unsuitable for use in the oral cavity as an ingredient of dental materials even if their amount is small.